Unfold a set of edges
I have created a script to do it.
There is probably a native way so I do not accept my own answer.  

Here is the script :

import bpy
import bmesh

obj = bpy.context.edit_object
mesh = obj.data
bm = bmesh.from_edit_mesh(mesh)

previous = 0
current = bm.verts[399] #index of one of the two extremity

points = [0]

flag = False
while flag is False:
    flag = True
    for edge in current.link_edges:
        nei = edge.other_vert(current)
        if nei is not previous:
            previous = current
            current = nei
            flag = False
            len = (previous.co-current.co).length
            points.append(points[-1]+len)
            break

for v in points:
    bm.verts.new((v,0,0))

bmesh.update_edit_mesh(mesh)